Methods of reducing a metal oxide by a carbonaceous material at sub-atmospheric pressures



y 1963 E. E. v. HEUN 3,096,174

METHODS OF REDUCING A METAL OXIDE BY A CARBONACEOUS MATERIAL ATSUB-ATMOSPHERIC PRESSURES Filed April 10, 1958 I& 5 /JA Flg.

m1 Z LL.L Z LL/ LL! J L/ LLL) INVENTOR Ellis Erik Vilhelm Helm ,WQ#M

A TTORNESG United States Patent This invention deals with processesinvolving the heattreatment of finely comminuted materials undersub-atmospheric pressures, as well as with means for carrying out suchheat treatment.

Examples of such processes are metal producing processes in which amixture of metal oxide and carbon is heated at sub-atmospheric pressuresto a temperature below the melting points of the metal oxide and of themetal produced. Another example is the decarburizing of powdered metals,particularly high-carbon ferrochromium, by the process of heating amixture of the metal to be decarburized and an oxidizing agent, whichlatter may consist of a layer of oxide on the individual metal particlesformed in a preceding oxidizing treatment. While the invention isprimarily concerned with processes of the types just indicated, it isnot limited thereto but may also be applied to processes in which acomminuted charge is to be heated at sub-atmospheric pressures in orderto effect, for instance, a degassing of the charge or a distillation ora sublimation of desirable products therefrom or a removal ofundesirable constituents.

At present, processes involving the heating in vacuo of a powderedmaterial have found very limited practical application because of thehigh costs. Conventional vacuum furnaces for large charges are expensiveto build and have a comparatively low production capacity, a long timebeing required both for heating the charge to the temperature requiredand for allowing the charge to cool to the temperature at which thefurnace can be opened and the charge removed therefrom.

The invention has for its principal object to provide a process for thevacuum heating of powders which can be carried out at low cost and inwhich the processing time per charge is comparatively short. Anotherobject is the provision of a process for the vacuum heating of powdersin which the furnace proper is allowed to operate at atmosphericpressure. Another object is to improve the transmission of heat to thepowder charge. Still another object is the provision of improved meansfor carrying out the process.

According to a principal feature of the invention, the finely comminutedmaterial to be heat-treated is charged into a sheet metal container thewall thickness of which is insuificient to enable the container toretain its shape against the atmospheric pressure on exhaustion atelevated temperatures, whereupon said container with the chargecontained therein are heated to the temperature required while asub-atmospheric pressure is maintained in the interior of saidcontainer. It will be noted that, in this method, the furnace properdoes not have to be operated at subatmospheric pressures. It willtherefore be possible to employ a furnace of any of the types usuallyemployed for metallurgical heat treating, for instance a resistancefurnace or a gas-fired furnace. As soon as the powder charge has beensubmitted to the treatment temperature the required time, the containercan be removed from the furnace and replaced by another containerenclosing a new charge to be heated. In this manner, the furnace can bemaintained continuously at its full operating temperature. The outerpressure will force the heated container wall inwards, so that a closeengagement between the container wall and the powder charge will bemain- 3,096,174 Patented July 2, 1963 tained, resulting in an efiicienttransmission of heat from the furnace chamber to the powder chargethroughout the heating period.

In this context, the expression powder or comminuted materials should beunderstood to comprise not only fine-grain powders, but also materialscomposed of comparatively large grains or particles, generally with aparticle size up to /2 inch. If the powder to be heattreated isfine-grained or contains a substantial proportion of fine grains, forinstance with a particle size below 0.5 mm. (0.02 in.), and is of such acomposition as to develop substantial quantities of gas on heating, itmay be necessary or advisable to shape the powder, with or without theadmixture of a binder, into pellets or other formed bodies, preferablyof a size not exceeding V2 inch. According to another possibility, thepowder with or without the addition of water or a binder is compressedor compacted in the container so as to form a more or less coherent,porous body which may be provided with one or more exhaust canalsserving to facilitate the discharge of gases or vapours from thematerial during the subsequent heat treatment, which in this case may bepreceded by a special drying treatment at lower temperatures serving toremove the Water or solvent, if such have been employed.

Regarding the choice of the shape and size of the sheet metal container,it is to be noted that the distance from any individual powder particleto the nearest portion of the container wall should be short, if a swiftheating of the entire charge to the temperature required is to beobtained. Preferably, therefore, the container is given the shape of aflattened case, the thickness of which may for instance be 5 in. orless.

In the accompanying drawings illustrating an embodiment of theinvention,

FIG. 1 is a cross-sectional view of a furnace enclosing a sheet-metalcontainer, and

FIG. 2 is a side view of the furnace, partially in section on the lineIIII of FIG. 1.

The furnace 1 is an electric resistance furnace of the bell type adaptedto be lifted and moved as required by means of a traversing hoist notshown. The furnace is shown in operative position enclosing a container2 shaped as a flat case of thin sheet metal, for instance 1.5 mm. (0.059in.) carbon steel sheet or silicon-chromium alloyed heat resistant steelsheet. The case is supported in an upright position on a stand 5 withside stanchions 6. The case is provided with a vent 8 connected to anoutlet tube 3 adapted to be connected to a vacuum pump (not shown). Inthe outlet tube and along the bottom of the casse are placed a number ofceramic sleeves 4, the purpose of which is to form an exhaust canal andto protect the tube 3 against collapsing under the action of the outerpressure.

The powder 9 to be heat-treated, for instance a partially oxidizedferrochromium powder, is charged into the container from above andcarefully packed by agitation of the container or by ramming. The lid isthen welded on, the container is put in position on the stand, and thetube 3 connected to the vacuum pump. After the bell furnace has beenlowered over the container, a protective gas is supplied to the furnacechamber through the tube 7. As soon as the material under treatment hasreached the required temperature or been held at high temperature for arequired period, as the case may be, the bell furnace is raised andpreferably put into operative position with regard to a chargedcontainer supported by another stand (not shown). To protect the firstcontainer against the oxidizing action of the air during the coolingperiod, a bell-shaped cover may be applied over the container as soon asthe bell furnace has been removed. The supply of protective gas throughthe pipe 7 may be allowed to continue for part of or all of the coolingperiod.

Instead of the ceramic sleeves 4, it is possible to employ other means,for instance a row or heap of irregular pieces of ceramic material or ofthe same material as the powder charge, in which case a; piece of metalnetting or a perforated sheet may be applied on the row or heap to"prevent the powder from filling the space between the irregular pieces.

It is also possible to employ a stationary furnace in the carrying outof the method according to the invention. The furnace may be adapted tothe simultaneous heat treatment of two or more containers. The exhaustoutlet may, if desired, be provided at the upper end of the container.To permit the removal of a container in red-hot condition without dangerof oxidization of the powder charge, provision may be made forhermetically closing the container while still under sub-atmosphericpressure, for instance by flattening the outlet pipe and applying a seamweld across the flattened portion. Provisions may also be made forfilling the container with a protective gas before removal from thefurnace.

In some cases it may be useful to subject the comminuted material to aheat treatment in a gas of atmospheric pressure before or after theheat-treatment at sub-atmospheric pressure. For instance, thedecarburizing heat treatment of ferro-chromium powder may comprise as afinal step the filling of the container with hydrogen at atmosphericpressure in order to remove possible rests of carbon and/or oxide. Ifrequired, this step may be repeated twice or more, the reaction productsbeing removed between the successive filling steps.

Before the treatment according to the invention, the charged containermay be subjected to a rinse with protective gas, so that the airremaining in the container will be replaced by the protective gas.

In a particular method according to the invention, the container isexhausted and hermetically closed before starting the heat treatment.Conveniently, said steps are carried out on the container while stilloutside of the furnace. This particular method will, of course, bepossible only when the charge has such a composition that thedevelopment of gas therefrom during the heat treatment will beinsignificant. An example is the heating of containers of stainlessaustenitic chromium-nickel steel sheet filled with a powder of the samekind of steel produced by granulating or atomizing liquid metal withsubsequent removal of the oxide layer on the grains or parto enable thecontainer to retain its shape against external atmospheric pressure onexhaustion at elevated temperatures, and heating said charged containerat atmospheric pressure While maintaining a sub-atmospheric pressure inthe interior of said container.

2. In the method of reducing a metal oxide by a carbonaceous material bythe heating of a comminuted solid mixture of's'aid materials atsub-atmospheric pressures, the steps of charging said unreacted mixtureinto a sheet metal container the wall thickness of which is insuflicientto enable the container to retain its shape against external atmosphericpressure on exhaustion at elevated temperatures, exhausting saidcontainer so as to produce a subatmospheric pressure therein,hermetically closing said container while maintaining saidsub-atmospheric pressure" therein, and heating said hermetically closedcontainer in a furnace chamber maintained at atmospheric pressure.

3.- In the method claimed in claim 1, the further steps of compressingthe mixture in the container prior to the exhaustion thereof, and ofproviding at least one exhaust canal in the body of compressed orcompacted material;

4. In the method as claimed in claim 1 the further step of shaping thecomminuted mixture into pellets before charging the mixture into thecontainer.

5. In the method as claimed in claim 1, the further step of compactingthe mixture in the container prior to exhaustion thereof.

References Cited in the file of this patent UNITED STATES PATENTS1,754,453 Balke Apr. 15, 1930 2,495,561 Wilson Jan. 24, 1950 2,725,288Dodds et a1 Nov. 29, 1955 l l l

1. IN THE METHOD OF REDUCING A METAL OXIDE BY A CARBONACEOUS MATERIAL BYTHE HEATING OF A COMMINUTED SOLID MIXTURE OF SAID MATERIALS ATSUB-ATMOSPHERIC PRESSURES, THE STEPS OF CHARGING SAID UNREACTED MIXTUREINTO A SHEET; METAL CONTAINER THE WALL THICKNESS OF WHICH ISINSUFFICIENT TO ENABLE THE CONTAINER TO RETAIN ITS SHAPE AGAINSTEXTERNAL ATMOSPHERIC PRESSURE ON EXHAUSTION AT ELEVATED TEMPERATURES,AND HEATING SAID CHARGED CONTAINER AT ATMOSPHERIC PRESSURE WHILEMAINTING A SUB-ATMOSPHERIC PRESSURE IN THE INTERIOR OF SAID CONTAINER.